15 Databases with Room

Overview of Local Databases in Android

When your app needs to store structured data that should survive app restarts and device reboots, a local database is often the best choice. Unlike simple key value storage or plain files, a database lets you organize data in tables, define relationships, and query it efficiently.

On Android, the underlying database engine is SQLite. It is a lightweight, relational database system that runs on the device. You can think of it as a single file on the device that contains tables and indexes, and that you access using SQL queries.

Working directly with SQLite in Android requires you to write a lot of boilerplate code. You need to handle database creation and upgrades, write raw SQL strings, map query results to your objects, and keep this mapping updated as your schema changes. This makes your code harder to maintain and more prone to subtle bugs.

This is where Room comes in. Room is part of Android Jetpack and provides an abstraction layer over SQLite. It still uses SQLite under the hood, but it lets you describe your database using Kotlin classes and interfaces instead of writing SQL everywhere.

Why Use Room Instead of Raw SQLite

Room targets common problems that appear with direct SQLite access. First, type safety. When you write raw SQL queries as strings, the compiler cannot verify if your column names are correct or if the query returns the expected types. Room checks your queries at compile time. If you rename a column or change a table structure in your code, Room can flag incorrect queries before you run the app.

Second, Room reduces boilerplate. Instead of manually opening database connections, creating cursors, and converting each row into Kotlin objects, you define data classes and data access interfaces. Room generates the necessary low level code for you.

Third, Room integrates well with other Android architecture components such as LiveData, Flow, and ViewModel. This makes it easier to build responsive, reactive UIs where the screen updates automatically when the underlying data changes.

Finally, Room provides a clear structure for your database layer. You define three main parts: entities, data access objects, and the database class. This consistent pattern makes your code base easier to navigate and to extend.

Core Building Blocks of Room

Room organizes your data layer into three main concepts that correspond to specific Kotlin constructs.

Entities represent the tables in your database. Each entity is usually a Kotlin data class annotated with @Entity. The fields in the data class map to the columns in the table. You mark one or more fields as primary keys, and you can configure other aspects like indexes and foreign keys.

Data Access Objects, often called DAOs, are Kotlin interfaces or abstract classes annotated with @Dao. They contain methods that define how you read from and write to the database. For example, methods to insert, update, delete, or query entities. You annotate these methods with @Insert, @Update, @Delete, or @Query. Room generates the implementation that runs the right SQL behind these methods.

The database class is an abstract class that extends RoomDatabase and is annotated with @Database. In this annotation you list all the entity classes that belong to the database and set the database version. Inside the database class you define abstract methods that return your DAO interfaces. Room uses this information to create the actual database at runtime and to link your DAOs to that database.

The interaction pattern is straightforward. Your app code gets a reference to the database class, accesses a DAO instance from it, and then calls DAO methods to perform database operations. Room handles the rest in the background.

Room and the Rest of Your App Architecture

In a typical Android app that follows the recommended architecture, Room lives in the data layer. Above it, you may have repositories that combine data from Room, from the network, or from other sources. Above those, ViewModels expose data to the UI.

Room is designed to work nicely in this kind of layered setup. DAO methods can return types that are easy to observe from the UI, such as LiveData or Kotlin Flow. When the database content changes, observers are notified automatically, and the UI can update without manual refresh logic.

Because Room insists on compile time validation and a clear separation of concerns, it encourages you to keep raw data operations inside the data layer and avoids mixing SQL logic directly into your activities or fragments.

Basic Usage Pattern

To use Room in a simple app, you follow a repeatable pattern. You define an entity class that describes a table, for example a Note with fields like id, title, and content. You define a DAO with methods to insert and retrieve notes. You create a database class that declares Note as an entity and exposes the NoteDao. At runtime, you build an instance of this database using the Room builder, usually as a singleton so that only one database instance exists per process.

Database calls should be done off the main thread to keep the UI responsive. Room enforces this rule by throwing an exception if you try to run certain operations on the main thread, unless you explicitly allow it. In practice, you combine Room with coroutines or other background mechanisms so that long operations do not block the user interface.

Versioning and Schema Evolution

Over the lifetime of an app, your data model often evolves. You might add a column, create a new table, or change some constraints. Room tracks the version of your database schema through the version field in the @Database annotation. When you change your entities in a way that affects the database layout, you increment this version. Room then knows that it must migrate the existing database on user devices to the new schema.

Handling these transformations is done through migration logic, which you can register with Room. Proper migrations ensure that users who install an updated version of your app keep their existing data consistent with the new schema. If a migration is missing or incorrect, Room can detect this problem and fail at startup rather than silently corrupt data.

Testing and Reliability Benefits

Because Room validates SQL at compile time and controls the database through a well defined API, it makes both unit testing and instrumentation testing more reliable. You can create in memory databases for tests, populate them with known data, and verify that your DAO methods behave as expected. This is much harder to achieve with hand written SQLite code that is spread across the app.

The combination of validation, structured APIs, and easier testing improves the overall reliability of your data storage layer. Problems such as mismatched column names or outdated queries become compile time issues instead of runtime crashes on user devices.

Summary of Room’s Role

Room is the recommended way to work with a local relational database in modern Android apps. It uses SQLite internally, but it replaces strings of SQL with structured, type safe Kotlin code. It defines a clear pattern using entities, DAOs, and a database class, and it integrates naturally with other Android architecture components.

As you learn more about Room, you will see how to describe entities in detail, how to write DAOs to express the operations you need, and how to manage database migrations when your schema changes. Together, these features let you build Android apps that handle structured local data in a robust and maintainable way.